1. Field of the Invention
The present invention relates to an optical pickup for recording or reading out record information to/from an optical disc, and more particularly, to an optical pickup of a compatible type which can be used for both of a DVD (Digital Video Disc) and a CD (Compact Disc).
2. Description of Background Information
The DVD is an optical disc which is capable of recording a large amount of digital information, and its recording scheme allows the recording of digital information such as motion picture, computer information, or the like on a disc having the same diameter of 12 cm as the CD, at a recording density of six to eight times as high as that of the CD. In order to achieve such a high density recording, various devices have been developed for the DVD. For example, one kind of the DVD called SD (Super Density Disc) will now be considered. In order to increase a recording capacity, the wavelength of a laser light source to be used is 650 nm (nanometers) or 635 nm, which is much shorter than 780 nm of the CD, and a numerical aperture NA of an objective lens is set to 0.6, which is larger than the corresponding value 0.45 of the objective lens for the CD. Furthermore, the MPEG2 is used as the data compression algorithm, thereby achieving a high density recording of about 5 G bytes (about eight times as high as that of the CD) on one side of the disc. As mentioned above, the recording is performed on the DVD at a density much higher than the CD. Therefore, it is necessary to set a spot diameter of a laser beam for reading pit information to a value that is fairly smaller than that of the CD. The spot diameter of the laser beam is proportional to the wavelength .lambda. of the laser being used and is inversely proportional to the numerical aperture NA of the objective lens. In the SD, such a structure is realized by using a laser light source of a short wavelength and an objective lens having a large numerical aperture NA.
In view of the point the DVD and the CD use the same recording format, it is desired that the CD can be also reproduced by a DVD player. When the wavelength .lambda. of the laser is shortened and the numerical aperture NA of the objective lens is raised for the DVD, however, the margin for the deviation angle of the disc surface from the direction perpendicular to an optical axis of the optical pickups that is, the so-called tilt angle, becomes small. Since the thickness of the disc of the Cp is larger than that of the DVD (for example, the disc thickness of one side of the SD is equal to 0.6 mm, that of the CD is equal to 1.2 mm), the spot diameter of the laser beam becomes enlarged in the CD due to a difference in aberration or the like. In such a situation, therefore, there occurs a problem that information of the CD cannot be read by using the optical pickup for the DVD.
In order to solve such a problem, there conventionally have been proposed such a method that two objective lenses for DVD and CD are prepared and the lenses are switched in accordance with the type of disc, a method that a correcting lens is inserted into a collimator portion, thereby correcting the aberration due to the disc, a method that a two-focal point lens using a hologram is used as an objective lens, and the like.
In the case of the method using two objective lenses or the method using the correcting lens, however, there are drawbacks that a complicated mechanism which occupies a large space is necessary, so that it is not suitable for a reduction in size. In the case of the method of using the hologram, drawbacks are such that since a diffraction and an interference are used and there are always a plurality of spots, the efficiency in use of a light cannot be raised. Furthermore, because of its multi-beam scheme, an influence by interference can develop easily.